This invention relates to an improved propulsion unit, and an improved method of cooling such a propulsion unit.
It is well known to provide propulsion units that are suspended below the hull of a vessel, typically a ship in order to provide the ship with propulsion, and such propulsion units are commonly referred to as PODs. The concept of a POD for ship propulsion has been known for some time (examples are shown in U.S. Pat. No. 5,403,216, and European Patent No. 1 010 614) and is now in common use. In such an arrangement, the propulsion motor, which is generally electrical, is contained in a pod-like housing suspended below the hull of the vessel. The motor is directly connected to one or more propellers at one end, or both ends, of the pod housing. In cases where there is a propeller at only one end, the propeller can be either in front of or behind the pod casing relative to the water flow.
It will be appreciated that as the ship moves, the POD suspended therebelow will experience drag, which will oppose the motion of the ship. There is therefore a desire to reduce the physical dimensions of the POD so as to minimize the drag experienced by the ship. Therefore, PODs generally have minimal access to the insides thereof, and the propulsion motor is generally mounted on, or in close proximity to the wall of the POD. Therefore, vibrations from the propulsion motor are readily transmitted through the wall of the POD, leading to noise being passed from the POD, into the surrounding water.
In many applications, it is desirable to minimize the level of noise transmitted to the surrounding water. A typical application requiring the minimization of noise is for cruise ships that want to travel into environmentally sensitive areas, environmental research vessels, fisheries research vessels, etc. However, it is a problem that known noise isolation systems tend to require an increase in the size of the POD, and that the design of the POD therefore tends to be a compromise between low noise and small size.
It is an aim of the present invention to overcome, or at least reduce, the problems discussed above.
According to a first aspect of the invention there is provided a propulsion unit arranged to propel a waterborne vessel comprising an electric motor, arranged to provide propulsion, and a housing, arranged to contain the motor, wherein said motor is mounted within said housing on resilient couplings.
An advantage of such an arrangement is that the vibrations from the motor to the housing are significantly reduced and, therefore, the noise emission from the propulsion unit is reduced. Previously, such propulsion units were not fitted with resilient couplings because they entailed making the housing larger (and thus less hydrodynamically efficient), or access to the couplings could not be provided due to the restricted access within the propulsion unit and, therefore, the couplings could not be maintained.
Preferably, the resilient couplings include metallic cushion elements, which are preferably woven metallic cushion elements. Such cushion elements are advantageous because they do not require frequent maintenance. In the most preferred embodiment metallic cushion elements are arranged to stiffen as the deflection of the cushion element increases. Such metallic cushion elements are available from Stop-Choc, of Banbury Ave., Slough, Berks, England.
It will be appreciated that the resilient coupling will have a natural frequency. In the preferred embodiment, the natural frequency of the resilient coupling is roughly at least twice the maximum supply frequency of the electric motor. Such an arrangement is convenient because the electric motor will generate vibrations due to the fundamental component of flux within the motor, which occurs at twice the fundamental supply frequency of the motor. It is advantageous to arrange that the natural frequency of the resilient coupling be greater than twice the maximum supply frequency to ensure that the resilient coupling does not amplify these vibrations, which would occur if the resonant frequency were roughly equal to twice the maximum supply frequency.
Preferably, the resilient coupling has a natural frequency roughly selected to suit the motor. Generally, this will be in the range of between roughly 20 Hz, and roughly 50 Hz. Of course, the resilient coupling may have a natural frequency other than this and may be roughly any one or more of the following (or any value in between): 5 Hz, 10 Hz, 15 Hz, 25 Hz, 30 Hz, 40 Hz, 50 Hz, 75 Hz. It will be appreciated that it is advantageous to have a low natural frequency because the resilient coupling will not attenuate frequencies below the fundamental frequency, and therefore, the higher the fundamental frequency, the less frequencies will be attenuated. However, if the natural frequency of the coupling is too low, then it does not provide enough stiffness, and deflections of the motor on the couplings become too large.
In one embodiment, the motor is an induction motor, although other types of electric motor, such as a synchronous motor, are possible.
The propulsion unit may comprise a pulse width modulated drive unit arranged to supply the motor. Such a drive unit is advantageous because the noise components that it introduces onto the current and voltage it supplies will generally be at a high frequency relative to the resonant frequency of the resilient coupling and such an arrangement is convenient because it allows these noise components to be readily attenuated by the resilient couplings. In general, during normal operation, the largest generation of vibration in the propulsion unit will be due to the non-sinusoidal components in the supply to the motor.
Preferably, the motor is provided with a limiting mechanism, arranged to limit movement of the motor relative to the housing. Such an arrangement is convenient in conditions in which the routine operating conditions of the motor are exceeded, e.g., fault conditions, or an external impact, etc. In such conditions, the resilient coupling may not be able to offer sufficient resistance to the movement of the motor, and thus, the limiting mechanism is desirable to prevent excessive movement of the motor.
The limiting mechanism may comprise a gap of predetermined dimensions between an abutment portion of the motor and an abutment portion of the housing arranged to co-operate with the abutment portion of the motor. Such an arrangement is convenient because it is structurally simple.
In the preferred embodiment the gap is roughly 1.0 to 1.5 mm. However, the gap may be any other suitable dimension, and may be, for example, roughly any one of following, or any dimension between any of the following: 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm. It will be appreciated that as the size of the gap increases, the more the motor will be allowed to move before its movement is stopped, and further the housing becomes larger to accommodate the extra gap. If the gap is made too small, there is more of a likelihood of the motor touching the housing and, thus, the propulsion unit is likely to emit more noise.
Preferably, a space is defined between the motor and the casing which is arranged to allow for the passage of cooling fluid around the motor. Such an arrangement is convenient because it helps keep the motor cooled. Generally, the fluid will be a gas, and in particular air.
In one embodiment, a plurality of resilient couplings is provided along a side region of the motor. The plurality of resilient couplings may be provided substantially along a line roughly parallel to the longitudinal axis of the motor. Preferably, in such an embodiment at least two lines of resilient couplings are provided, preferably roughly diametrically opposed to one another. Such an arrangement is convenient because it may be more compact than other possible arrangements.
The housing may have extended portions arranged to house the resilient couplings.
An intermediate member may be provided between the housing and the resilient couplings. The intermediate member may comprise a bar running substantially parallel to the axis of the motor. An intermediate member may be advantageous because it may allow for easier alignment of the resilient members with the housing.
In an alternative, and perhaps less preferred embodiment, the resilient couplings may be provided at end regions of said motor. Preferably, a plurality of resilient couplings is provided at each end region thereof. Such an arrangement is convenient because it may provide for easier construction of the propulsion unit, but may result in a larger unit.